OCI zstd

A friend of mine wrote https://github.com/jonjohnsonjr/targz, which is essentially extracted from the code that powers the layer browsing functionality of https://oci.dag.dev/ (https://github.com/jonjohnsonjr/dagdotdev). 👀

My understanding of oci.dag.dev is that he creates an index of the tar inside the stream (without modifying the original compression in any way). Then he gets clever and stores that in a tar.gz so that if the *index* gets too big, he can make a map of the index too and just recurse.

(However, my own understanding of the details is very surface level, so if I've got the details wrong maybe he'll finally make an account just to correct me! ❤️)